1. Field of the Invention
The present invention relates to antifuse technology. More particularly, the present invention relates to electrically programmable metal-to-metal antifuse structures which are tolerant to read-disturb phenomena and to processes for fabricating these antifuse structures.
2. The Prior Art
Integrated circuits which can be configured or programmed by the user for a specific application are becoming increasingly popular. Programmability of such circuits is accomplished by employing programmable interconnect elements in the packaged integrated circuit which the user programs to make or break connections to selected electrical nodes within the circuit such that the programmed device may perform a desired function.
Antifuses are used extensively in user-programmable circuits and are well known. Antifuses generally consist of two conductive electrodes separated by one or more layers of insulating material. The lower conductor is covered with an insulating material into which an antifuse aperture is disposed in the region where it is desired to form the antifuse. The antifuse material is disposed in the aperture region and is covered by one or more of conductive layers which will serve as the upper electrode. Unprogrammed, these links isolate the electronic nodes in the programmable device. The resistance of an unprogrammed antifuse is typically in the range of 1 Gohm.
When programmed an antifuse creates a relatively low resistance link between two circuit nodes in the typical user programmable device. During programming, the antifuse material between the electrodes is broken down by a current developed from a predetermined programming voltage (typically 16-20 volts) applied to the electrodes of the selected antifuse. A conductive filament is thereby formed which electrically connects the upper and lower electrodes of the selected links. The resistance of a programmed antifuse is less than 1 Mohm and typically in the range of hundreds or thousands of ohms.
In most applications, it is desirable to make the resistance of a programmed antifuse as low as possible in order to maximize circuit performance, and to minimize drive, impedance-matching and noise problems within the circuit containing the antifuse. In addition, antifuse reliability is crucial to providing a commercially-viable product. In any circuit containing a plurality of antifuses, the antifuse links must be manufactured so as to reliably program at the selected programming voltage and to remain in their user-selected state during the operating life of the circuit of which they are a part. This requires minimizing the leakage currents that flow in unprogrammed antifuses and assuring that programmed antifuse do not change resistance over time. The conductive filament of a programmed antifuse must offer high electromigration immunity, thereby maintaining the low resistance conductive path for the operating life of the device.
There are several different varieties of antifuses. One type of antifuse includes a lower electrode comprising a doped region in a semiconductor substrate and an upper electrode comprising a heavily doped polysilicon layer. A multilayer sandwich structure with oxide-nitride-oxide (ONO) comprises the antifuse material. Examples of these structures are found in U.S. Pat. Nos. 4,823,181 to Mohsen et al. and 4,899,205 to Hamdy et al.
Recently, another type of antifuse formed between metal electrodes which may comprise portions of metal interconnect layers disposed in the integrated circuit has been employed. The metal-to-metal antifuse offers the advantage of low programmed antifuse resistance as well as the ability to fabricate denser antifuse array architectures due to the fact that no substrate area is needed for the antifuse structure.
Despite the advantages of metal-to-metal antifuses, it has been observed that they are susceptible to a phenomenon known as "read-disturb," which is defined as an increase in the ON resistance of the programmed antifuse during its lifetime. ON resistance of the antifuse may even increase to the point where the antifuse will revert to its open-circuit state. Read-disturb is obviously a substantial reliability issue.
After the antifuse is programmed it has to endure operation conditions where it could be subjected to both DC and/or AC voltage pulses at both high and low temperatures. During operation, when a DC voltage with a polarity opposite to the polarity of the programming pulse is applied to the already programmed antifuses, the ON state of the antifuse can be disturbed and the ON resistance can increase, sometimes to the point where the antifuse reverts to its OFF (unprogrammed) state again.
It is believed that the read-disturb phenomenon is largely an artifact of electromigration of the conductive filament material. It has been observed that during positive DC stress, the on resistance can be disturbed, however, the probability is much less than the reverse DC stress. The read-disturb phenomenon is also observed under AC operating conditions. In addition, when the operation temperature is raised, the probability of read-disturb will increase.
In an attempt to minimize the effect of read-disturb on metal-to-metal antifuses, one current prior art approach has been to limit their operating current to a value far below the programming current. The idea is that with high programming current, the conductive link created is much larger. With low operating current, the disturb is small since only a small part of the link will be affected by the low current. However, the penalty of such an approach is that in order to provide high programming current, the transistor in the integrated circuit used to provide the programming current to the antifuse has to be made larger. This increases the die size and limits the ability to scale the integrated circuit device containing the antifuses.
A second approach employed by the prior art in an attempt to overcome the read-disturb problem is to reduce the thickness of the layer of antifuse material. By reducing the antifuse thickness, a larger conductive filament can be created with the same programming voltages. The larger filaments will be less susceptible to read-disturb problems. However, when this approach is used, the antifuse may exhibit poor leakage, higher capacitance and increased failure characteristics.
In order to overcome the disadvantages of the prior art antifuses while maintaining low capacitance, and better breakdown voltage and leakage characteristics, it is the object of the present invention to provide an electrically programmable antifuse composite structure which improves the "read-disturb" resistance without having to increase the programming current. This involves depositing a highly conductive material, such as metals or doped amorphous silicon, sandwiched inside the antifuse composite.
It is therefore an object of the invention to provide a metal-to-metal antifuse structure which has an improved immunity to read-disturb problems.
It is a further object of the invention to provide a process for fabricating a read-disturb resistant antifuse.